This invention relates to the field of automatic transmissions for motor vehicles. More particularly, the invention pertains to the kinematic arrangement of planetary gearing, clutches, brakes, and overrunning couplings for such transmissions.
Minimizing the package size of a geared automatic transmission for motor vehicles, its lateral dimensions and particularly its axial dimensions, has long been an objective in the automotive industry. Realizing this goal has become more difficult because of the need for transmissions to provide a continually increasing number of forward gear ratios and a need for non-synchronous shifting among the gear ratios. Automatic transmissions having five and six forward gear ratios are replacing current transmissions having four and five forward gear ratios.
In order to minimize the axial length of an automotive transmission, the number of friction clutches and brakes is minimized. It has become conventional to provide non-synchronous gear ratio changes, especially among the lower gears, by providing a one-way coupling in parallel with a hydraulically actuated friction clutch or brake. However, a one-way coupling requires additional space along the axis of the transmission. Its presence also adds to assembly time, material cost, and weight.
In addition to minimizing the package size of automatic transmissions, it is important also to minimize the rotational speed of the pinion gears, which are supported on a carrier of the planetary gear units within the transmission. Excessive planet pinion gear speed can adversely affect the service life of the bearings on which the planet pinions are supported for rotation on the carrier. It is not uncommon that the rotational speed of planet pinions of certain gear units be five to seven times the engine speed.